Multiple sudden coolant losses

Hi all, I'm driving an old (1998) Rover 214 and it's been loosing coolant for a while.

There are no apparent leaks, they are sudden losses of maybe 0.2 - 0.4 liters. This has happened at least five times (only once I actually saw the coolant spill on the floor).

When I open the tap of the reserve tank I notice quite a bit of pressure and some coolant flows back up. I took it to the shop and they said it may be a head gasket, but I wonder if it could be simply the pressure cap or something else? I don't want to spend a load of money as the car is almost ready to retire anyway.

Air bubble in the system?

Air bubble in the system?

Could be bubbles. How do I find out? How did the bubbles got in (I hadn't changed or refilled the coolant a several months before this started happening)? How do I get the bubbles out?

My guess is a head gasket.
Air bubbles cause overheating problems not coolant leaks.

My guess is a head gasket.
Air bubbles cause overheating problems not coolant leaks.

How much would they charge me to have the head gaskets inspected
and replaced if needed?

Water pump.